(p). Physical and Chemical Characteristics of Seawater

Seawater is a mixture of various salts
and water. Most of the water in the ocean basins is believed
to originate from the condensation of
water found in the early atmosphere as the Earth cooled
after its formation. This water was released from the lithosphere as
the Earth's crust solidified. Additional water has also
been added to the oceans over geologic time from periodic volcanic action.
Some scientists have recently speculated that comets
entering the Earth's atmosphere may be another important
source of water for the oceans.

Most of the dissolved chemical constituents
or salts found in seawater have a continental origin.
It seems that these chemicals were released from continental
rocks through weathering and
then carried to the oceans by stream runoff.
Over time, the concentration of these chemicals increased
until an equilibrium was met. This equilibrium occurred
when the ocean's water could not dissolve any more material
in solution. Similarities between fossilized sea life
and organisms living today indicate that the composition
of seawater stopped changing drastically about 600 million
years ago.

Only six elements and
compounds comprise about 99% of sea salts: chlorine
(Cl-),
sodium (Na+), sulfur (SO4-2),
magnesium (Mg+2), calcium (Ca+2),
and potassium (K+) (Figure
8p-1). The relative abundance of the major
salts in seawater are constant regardless of the ocean.
Only the amount of water in the mixture varies because
of differences between ocean basins because of regional
differences in freshwater loss (evaporation) and gain
(runoff and precipitation).

The chlorine ion makes up 55% of the salt
in seawater. Calculations of seawater salinity are made
of the parts
per 1000 of the chlorine ion present in one kilogram
of seawater. Typically, seawater has a salinity of 35
parts per thousand.

Figure 8p-1: Relative
proportions of dissolved salts in seawater.

Water is one of the few substances existing
on the Earth's surface in all three forms of matter.
At zero degrees Celsius liquid water turns into ice and
has a density of approximately 917 kilograms per cubic
meter. Liquid water at the same temperature has a density
of nearly 1,000 kilograms per cubic meter. The density
of seawater generally increases with decreasing temperature,
increasing salinity, and increasing depth in the ocean.
The density of seawater at the surface of the ocean varies
from 1,020 to 1,029 kilograms per cubic meter. Highest
densities are achieved with depth because of the overlying
weight of water. In the deepest parts of the oceans,
seawater densities can be as high as 1,050 kilograms
per cubic meter.

Seawater freezes at a temperature that
is slightly colder than fresh water (0.0° Celsius).
The freezing temperature of seawater also varies with
the concentration of salts. More salt the lower the initial
freezing temperature. At a salinity of 35 parts per thousand,
seawater freezes at a temperature of -1.9° Celsius.

Sea ice normally contains considerably
less salt than seawater. Most of the salts found in liquid
seawater are forced out it when freezing occurs. The
reason for the exclusion is because the molecules of
the various salts do not fit well in the highly orderly
molecular structure of frozen water. Because of the density
difference between ice and seawater, ice floats on the
surface of the ocean.

Seawater also contains small amounts of
dissolved gases. Many of these gases are added to seawater
from the atmosphere through the constant stirring of
the sea surface by wind and waves. The concentration
of gases that can be dissolved into seawater from the
atmosphere is determined by temperature and salinity
of the water. Increasing the temperature or salinity
reduces the amount of gas that ocean water can dissolve.
Some of the important atmospheric gases found in seawater
include: nitrogen, oxygen, carbon dioxide (in the form
of bicarbonate HCO3), argon, helium, and neon.
Compared to the other atmospheric gases, the amount of carbon
dioxide dissolved in saturated seawater is unusually
large.

Some gases found within seawater are also
involved in oceanic organic and inorganic processes that are indirectly
related to the atmosphere. For example, oxygen and carbon
dioxide may be temporally generated or depleted by such
processes to varying concentrations at specific locations
within the ocean.